This invention relates to an application method of a coating fluid and more particularly to an application method using an extrusion type coating head for applying a uniform coating of fluid extruded from a slot end to the surface of a flexible support consecutively running in support of run guide means such as guide rollers with the coating head nose directed toward the flexible support surface.
Hitherto, a step of applying a desired coating fluid to the surface of a flexible support has been executed in a manufacturing process of photo-sensitive material, magnetic recording media, etc. Coating fluids containing various fluid compositions are available according to their applications, for example, coating fluids of photo-sensitive emulsion coats, undercoats, protective coats, back coats, etc., in photo-sensitive material and those of magnetic coats, undercoats, protective coats, lubricant coats, etc., in magnetic recording media. The coating fluids are aqueous and organic solvent solutions, etc., containing their indispensable components, binders, and various admixtures as required.
Hitherto, various application methods, such as a roll coat method, a gravure coat method, a roll coat plus doctor method, extrusion type application method, and slide coat method, have been used as methods of applying such coating fluids to the flexible support surface. In recent years, the application method using an extrusion type coating head has been often used for applying a magnetic coating fluid.
A typical method using an extrusion type coating head is, as well known, a method of applying a thin and uniform coating of fluid extruded from the slot end to the surface of a flexible support consecutively running with the coating head disposed between a pair of run guide means spaced at a given distance apart at a predetermined place on which the flexible support is placed.
The method using the extrusion type coating head includes, for example, a so-called pressurizing type application method of directing the nose of a coating head, namely, the top of the front edge and back edge (doctor edge) defining a slot from which a coating fluid is extruded toward the surface of a flexible support mounted on a pair of run guide means, guide rollers and consecutively running in support of the guide rollers so as to press it against the flexible support surface and changing the space between the nose and the flexible support surface in response to a change in the extrusion amount of the coating fluid for applying the coating fluid under pressure to the flexible support surface.
To solve disadvantages of the pressurizing type application method, such as being easily affected by uneven thickness of the support and an unstable state in which behavior of coating fluid is easily placed out of order by coating fluid pressure, a method as disclosed in U.S. Pat. No. 4,907,530 was proposed, wherein, as shown in FIG. 4, a coating head 1 has a slot 6 extending from a fluid reservoir 8 to the coating head nose, an edge face 30 of a front edge 3 is curved so as to swell toward the side of a flexible support 2, and the tip 41 of an edge face 40 of a back edge 4 recedes from the end 31 of the front edge face with a level difference in the direction opposite to the flexible support. Therefore, the edge face 40 of the back edge 4 enables a coating fluid 5 extruded from the slot 6 to be applied to the support in a natural flow without being pressed against the support side. That is, foreign material such as accompanying dust particles adhering to the support are not trapped on the edge face 40 of the back edge 4 and air is not involved in the support either, thereby enabling high speed application and efficiently preventing "vertical stripes" along the running direction of the support from occurring on the coat.
According to the non-pressurizing type application method, a thin coating can be applied as doctor blade type application, but the method requires improvement to meet demands for high speed, thin coating, and high density and moreover stable mass application.
Then, further research shows that high speed thin coat application performance of a coating fluid applied to a precoat depends greatly on the precoat thickness at extremely high speed application.
However, hitherto, the precoat fluid thickness has been controlled almost depending on experience. A problem remains unsolved on stable application at thin coating application at high speed such as 400 m/min or higher. As a result of intensive research, a problem hindering high speed thin coating application due to film cut of a coating fluid in relation to coating conditions has been highlighted to advance high speed thin coating application.